Conventional security keypads are alpha-numeric and usually arranged in some type of matrix configuration, e.g. row-column configuration. Such keypads are used to authorize or verify the identification of an individual seeking to gain access to certain areas or devices. It will be appreciated by one ordinarily skilled in the art, however, that keypads may be used for other purposes as well.
Typically, an individual will be given an access code that must be entered into the keypad to receive authorization. The keypad usually has a set of row conductors, one conductor corresponding to each row, and a set of column conductors, one conductor corresponding to each column. When a particular key is depressed, an electrical connection of that key's row and column conductor is formed and a signal is sent to a processor which can identify the depressed key. Identification is accomplished by having the processor interrogate the rows and columns to see if any are in an active state, e.g. if a specific signal is present. An active state can only be achieved when a key is depressed which causes a row conductor to physically contact a column conductor thereby creating a closed circuit.
The processor interrogates the rows and/or columns continuously in order to determine when keys are depressed. Most processors will perform the interrogation in a particular non-varying sequence. A problem that may arise under such an interrogation process is that certain electronic devices are capable of eavesdropping on an individual as the individual depresses the keypad. This is possible because electromagnetic radiation is emitted by the keyboard when a key is depressed. The characteristics of this emitted radiation vary when a row or column is interrogated which has a key depressed. Further, the characteristics of this emitted radiation also differ depending on which key in the row or column is pressed. Consequently, it is possible for an individual possessing electronic eavesdropping equipment to detect the emitted radiation from the keypad and process same to determine the exact keys pressed and the sequence in which they were pressed.
One solution to the above described eavesdropping problem is to randomly interrogate the rows and/or columns. Random interrogation prevents an eavesdropper from establishing a repetitive baseline pattern needed to compare the detected radiation against. Without a baseline pattern it becomes impossible for an eavesdropper to correspond a certain distinct level of emitted radiation with a specific row and/or column.
U.S. Pat. No. 5,025,255 to Mould describes a secure keyboard with random order of interrogation in which the processor associated with the keyboard generates a random number which is then associated with a particular row to be interrogated. In the preferred embodiment, this process is repeated until all of the rows have been interrogated. This is a computationally taxing method since, after each random number is generated, a check must be performed to determine whether the row associated with that random number has already been interrogated, and, if so, a new random number must be generated and checked again until a random number corresponding to a row that has not been selected is returned. As the number of rows dwindles, the odds of a random number corresponding to a row already selected increase substantially. Thus, significantly many more random number generations than number of rows may be required to finish one complete set of row interrogations.
A second embodiment of U.S. Pat. No. 5,025,255 to Mould describes selection of an entire sequence of rows to be interrogated all at once rather than generating a random number for each row. This embodiment requires that each entire sequence be stored in memory. Thus, as the number of columns grows the amount of memory required to store the interrogation sequences grows exponentially.
A third embodiment of U.S. Pat. No. 5,025,255 to Mould describes using a random number generator to generate a random number which corresponds to a memory location containing an interrogation sequence. This embodiment suffers from the same shortcomings as the second embodiment, namely, a large memory requirement.